Paper-based microfluidic devices have emerged as a platform that is capable of supporting the development of a number of useful analytical and bioanalytical sensors, the capabilities of which range from the detection of environmental contaminants to metabolites in blood plasma. Paper-based sensors often produce colorimetric results, which allow data to be interpreted rapidly at the point-of-use in a manner that is either qualitative (i.e., by eye) or quantitative through the use of simple readers. Utilizing paper as a substrate to develop analytical assays is beneficial because the infrastructure required to produce the analytical assays is minimal (e.g., a printer, heating element, and pipette), raw materials are inexpensive and ubiquitous (cents per sheet), and devices can be prototyped rapidly (within minutes from conception to use). By patterning paper with hydrophobic barriers, hydrophilic channels can be designed to control the wicking of fluids by capillary action. Complex, three-dimensional microfluidic networks can be fabricated from either stacking multiple layers of paper or folding a single layer of paper (i.e., origami). Simple design rules provide access to many different architectures of fluidic networks, which can facilitate the manufacture of devices that range from one-step, field-deployable diagnostic tools to sophisticated paper “machines.”
With the considerable interest in this field of research, a glaring oversight has been the lack of applications of paper-based microfluidic devices for the separation or detection of cells. This omission appears to be caused based on a perspective in which paper is viewed as a passive substrate, instead of being viewed as a component that is fundamental to the function of the microfluidic device. Consequently, paper has been applied only to the filtration of all cells from plasma or to the separation of misformed (i.e., sickled) red blood cells, or as a scaffold for the study of cultures of mammalian cells. However, the ability to detect cells has significant value in applications related to, among others, personalized healthcare, monitoring of livestock, and determining the quality of food and water. These important capabilities are currently only available in established economies with centralized laboratories that are equipped with modern instrumentation and that include an educated workforce. Consequently, a significant percentage of the world's population—particularly those living in low-income and middle-income countries—have limited access to tools that could drastically improve the quality of life. Accordingly, paper has the potential to enable new classes of biological separations, analytical sensors, and point-of-use assays for underrepresented populations across the globe.